Welding robots now come into wide use since they fulfill objectives such as labor saving, quality-enhancement or improvements in environmental conditions of welding work or the like, in place of conventional welding work relying upon experiences or intuition of an operator. Among welding operations performed by the welding robot, multi-layer welding is a very important operation for reasons such as securing a strength of a welded portion but robots have many technical difficulties.
Japanese Patent Application Laid-Open (Kokai) No. 58(1983)-188572 (corresponding to U.S. Pat. No. 4,508,953) discloses a multi-layer welding operation, generally, the first layer is welded according to the tracking function, and therefore, a deviation occurs between a teaching path and an actual welding path. Even if a teaching path is used as a reference path for determining paths for the second and succeeding layers, the tracking function is necessary for the second and succeeding layers similar to the first layer. However, in the second and succeeding layers, the first layer is already filled with a bead, and therefore, a current variation cannot be obtained. Accordingly, groove position information is not known.
For this reason, a method which uses no tracking function for the second and succeeding layers stores an actual welding path when the first layer is welded and determine paths for the second and succeeding layers on the basis of the stored path. In this method, there is a method for corresponding the locus when the first layer is welded to instructed points of the teaching path to store it as a point having passed during the actual welding and generating an actual welding path at the stored point, as proposed in the aforesaid Japanese Patent Application Laid-Open (Kokai) No. 58(1983)-188572.
However, when generally, the tracking function is used, a welding torch follows an actual welding line with unevenness under the tracking conditions, and therefore, in the aforesaid method, case where the actual welding path is defined merely on the basis of the point having passed during actual welding, that is, in the case where the instructed points are defined by two passed point, said unevenness becomes included in data of the passed points, and as a result, an error occurs between original welding lines.
More specifically, as shown in FIG. 1, an error occurs in the obtained actual welding path (indicated at the broken line between x and x) irrespective of the presence or absence of a positional deviation between the instructed path and the actual welding line (a curve indicated by the solid line). Accordingly, if the actual welding path is defined by the conventional method to determine paths of the second and succeeding layers using the actual welding path as a reference, an error in the first layer becomes included in the final layer.
As the way for obtaining an operational locus of the second and succeeding layers, Japanese Patent Application Laid-Open (Kokai) No. 58(1983)-187270 discloses a method for storing the operational locus of the first layer in a memory of a robot control device, providing a shift width .DELTA.S as a parameter in an arithmetic processor of the robot control device to obtain the shift direction of the second and succeeding layers from the stored data of the first layer, and obtaining the operational locus of the second and succeeding layers from said shift width to carry out the multi-layer welding.
The aforesaid method is a method comprising teaching within an X-Y plane and having a restriction of setting of a shift amount which is accurate and only in one direction.
However, actually, in the welding operation by the robot, there are present three-dimensional errors due to reasons such as occurrences of an error in work setting, a thermal strain during welding, an error in temporary welding and the like, and therefore, the welding locus cannot be defined on the X-Y plane. Further, the shift direction of the multi-layer has to be considered in terms of the three dimension instead of the two-dimension.
From the foregoing, it is impossible to perform multi-layer welding by use of the aforesaid method.
Japanese Paten Application Laid-Open (Kokai) No. 57-50279 discloses a welding robot for calculating a position of the tip of a welding torch from a predetermined constant and variables so that the tip of a welding torch can be controlled to be moved along the desired locus, wherein the predetermined constant is sequentially changed by a preset amount, and the tip of a welding torch is moved by said preset amount from the desired locus to render multi-layer welding possible.
However, in this welding robot, a biaxial angle of a wrist portion of the robot is obtained from the shift amount to perform positional control, and therefore, the shift amount at the tip of a welding torch is limited by a mechanism of the robot. Accordingly, this welding robot is not for general purpose. Furthermore, since the wrist is constituted by two axes, the freedom in the attitude of the torch which is essential to welding is poor (the wrist needs to have at least three freedoms in order to increase the freedom).
Next, as a method for calculating the attitude of the torch of the second and succeeding layers, Japanese Patent Application Laid-Open (Kokai) No. 57-50279 discloses a method for multi-layer welding for calculating a position of the tip of a welding torch from a predetermined constant and variables so that the tip of a welding torch may be controlled to be moved along the desired locus, wherein the desired constant is sequentially changed by a preset amount, and the tip of the welding torch is moved by said preset amount from the desired locus to thereby render multi-layer welding possible.
However, it is sometimes requested in terms of execution of welding that the attitude of the torch 7 is changed sequentially in the second and third layers in an attitude different from that of the first layer as shown in FIG. 2. In such a case, in the aforesaid method for multi-layer welding, calculation will be made of the position alone, and the change of the attitude of the torch 7 is not taken into consideration. Therefore, in the case where the attitude of the torch 7 has to be changed, correction needs be made sequentially in the second and the third layers by teaching, taking considerable time.